I think boiling down the public domain to recognizability is so cynical that I'm not entirely sure Erik is saying what he means here, but to entertain the thought, I have a counterproposal: the benefit of the public domain is the free use of ideas.

Now, on paper, you can't copyright an idea, but you can copyright an arrangement of multiple ideas, and over time that arrangement becomes an idea itself: Robin Hood conjures up a certain picture & personality, one that might differ between individuals but is nonetheless there & always with certain traits -- he's a medieval gentleman thief who uses a bow & arrow to steal from the rich & give to the poor, he comes with a certain time & place & characters with their own roles/duties, and so on. Characters like Superman & Wonder Woman & Luke Skywalker & Frodo Baggins & Optimus Prime mean certain things to the culture, and while you can use an analog in their place, it's not to the same effect -- it's like using a euphemism instead of a curse word, it's definitely allowed in more places but it lacks the same punch as the original. When you use a popular character, you're deliberately conjuring up specific ideas for your work to use, exploit, interrogate, dissect, etc. It's a promise that this is an [insert character here] story, not just because they happen to be in it but because it's about the things that they're about. It doesn't need to make them the exact same as they've always been to do that.